The demand for high resolution, high frame rate, high dynamic range and low noise CMOS image sensors continues to push the technology of our pixel readout circuitry. For next generation CMOS image sensors, we see an increase requirement for higher resolution ADCs pushing up towards 16 bits.
The need for low noise, high bit resolution becomes especially important in low light levels, where the readout noise may not be dominated by photon shot noise.
In such CMOS image sensors, the signal noise is dependent on the signal level. For example, for CMOS image sensors, the noise is proportional to:Noise=√{square root over ((readnoise)2+(signal))}{square root over ((readnoise)2+(signal))}
Note that this noise value is typically effective at all signal levels. The signal dependent portion of the noise is called the “shot noise”.
Because of this relationship, the ADC used to digitize the light can have a lower resolution at higher light levels than it has at lower light levels.
FIG. 1 shows how ADC quantization noise hence scales with the input signal. Specifically, as shown in FIG. 1, the quantization noise shown as 100 scales upward as the input voltage to a successive approximation A/D converter increases. For example, the section 105 of the curve shows the quantization noise for a nine bit A/D conversion. This is more or less linearly increasing as the input voltage increases. In a similar way, the section of the curve 110 shows how the quantization noise scales upward in a higher bit A/D converter.